Compensation of delay in linearization loop of power amplifier

ABSTRACT

A method of compensating delay in an linearization loop of a power amplifier and a linearization arrangement of a power amplifier, which arrangement includes an I/Q modulator, one or more power amplifiers to be linearized and generating delay, and a feedback loop including an I/Q demodulator when the I/Q modulator and the I/Q demodulator derive an oscillator frequency from the same local oscillator. In accordance with the invention, the delay generated the power amplifier in the feedback is compensated by delaying a local oscillator signal applied to the I/Q demodulator. The delay of the local oscillator signal applied to the I/Q demodulator is generated by an amplifier optimized to have a great delay, preferably by a small-signal amplifier.

This application is the national phase of international applicationPCT/F198/00786 filed Oct. 7, 1998 which designated the U.S.

BACKGROUND OF THE INVENTION

The present invention relates to a method of compensating delay in alinearization loop of a power amplifier, which loop comprises an I/Qmodulator, one or more power amplifiers to be linearized and generatingdelay, and a feedback loop comprising an I/Q demodulator when the I/Qmodulator and the I/Q demodulator derive an oscillation frequency fromthe same local oscillator.

Linearized amplifiers are needed in current digital wirelesscommunications systems, for example, since it is required that thespectrum of a signal to be transmitted must not spread broader than theactual useful band. The spreading of a spectrum is caused by thenonlinearity of amplifiers and it generates interference to neighboringchannels, for example. The linearity of amplifier stages depends on howthey are biased and they can be classified according to linearity: aclass A amplifier is the most linear but it has poor efficiency, while aclass C amplifier, for example, has good efficiency but it is alsohighly nonlinear. Good efficiency is an important property in poweramplifiers, and this is particularly emphasized in wirelesscommunications devices whose battery capacity is limited. Hence,amplifiers that have good efficiency but are nonlinear and must belinearized are used.

A known method of linearizing a nonlinear radio-frequency poweramplifier is cartesian feedback. In outline, its operational principleis as follows: the data to be transmitted is included in the basebandsignals I and Q. These signals are conveyed to an I/Q modulator in whichthe signals are combined and modulated directly to a final frequency. Afinal-frequency signal is amplified by one or more nonlinearradio-frequency power amplifiers and conveyed to an antenna. Theradio-frequency, amplified signal is sampled after the last amplifierstage by a directional coupler, for example. The sample signal isconveyed to an I/Q demodulator in which it is demodulated to basebandand the I and Q signals are separated from it. The baseband I and Qsample signals are finally summed to the actual I and Q signals. Thesumming generates predistortion of the I and Q signals, on account ofwhich predistortion the nonlinearity generated in the power amplifiersis at least partly cancelled.

A problem in the arrangement described above is that the I/Q modulatorand the I/Q demodulator both receive a local oscillator signal from thesame source. On the other hand, the power amplifiers generate delay andthe sampled I and Q signals return in a wrong phase. This distortion canbe compensated by adjusting the phase of the local oscillator signalapplied to the I/Q demodulator to correspond the phase of the I and Qsample signals. In the known solutions this phase adjustment isimplemented by transformers, by special phase inversing circuits or adigital phase adjuster. The known solutions are similar in beingspace-taking and relatively expensive. Furthermore, the solutions areoften complex and may require much manual work in production. Suchsolutions are thus rather poorly suited to small mobile communicationsdevices produced in great numbers.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is thus to provide a method and an apparatusimplementing the method so as to solve the above-mentioned problems. Theobjects of the invention can be achieved by a method of compensatingdelay in a linearization loop of a power amplifier, which loop comprisesan I/Q modulator, one or more power amplifiers to be linearized andgenerating delay, and a feedback loop comprising an I/Q demodulator whenthe I/Q modulator and the I/Q demodulator derive an oscillationfrequency from the same local oscillator, whereby the delay generated bythe power amplifiers in the feedback is compensated by delaying a localoscillator signal applied to the I/Q demodulator, whereby the method ischaracterized in that the delay of the local oscillator signal appliedto the I/Q demodulator is generated by an amplifier optimized to have agreat delay, preferably by a small-signal amplifier.

The invention is based on the idea that delay is compensated by delay,not by phase adjustment. In other words, an arrangement which causesphase shift is replaced by an arrangement which generates delay in thesignal branch applied to the I/Q demodulator of a local oscillator. Anadvantage of the solution is that the arrangement is fairly simple. Inaccordance with the invention, the delay of a local oscillator signalapplied to the I/Q demodulator is generated by an amplifier optimized tohave a great delay, preferably by a small-signal amplifier. The groupdelay, i.e. the delay, of the amplifier is preferably 5 to 10 ns ormore. Since the local oscillator signal must usually be amplified beforethe I/Q demodulator in any case, the method of the invention can easilybe used by optimizing an existing signal amplifier to have a delay. Theadvantage is that no additional phase adjustment circuits are needed.

The invention further relates to a linearization arrangement of a poweramplifier, which arrangement comprises an I/Q modulator in which thebaseband signals I and Q containing data are combined and modulated to afinal frequency, one or more power amplifiers to be linearized andgenerating delay, by which power amplifiers a final-frequency signal isamplified and which signal is conveyed to an antenna to be transmittedafter the amplification, and a sampling arrangement by which theamplified, final-frequency signal is sampled before the antenna, and anI/Q demodulator to which said signal sample is applied and in which thesignal sample is demodulated to base-band and I and Q sample signals areseparated from it, and a feedback in which the I and Q sample signalsdemodulated and separated from the signal sample are summed to theactual I and Q signals, and a local oscillator from which a localoscillator signal is conveyed to the I/Q modulator and the I/Qdemodulator, whereby an arrangement is provided between the localoscillator and the I/Q demodulator, by which arrangement the localoscillator signal applied to the I/Q demodulator is delayed in order tocompensate the delay generated in the power amplifiers, whereby thelinearization arrangement is characterized in that the arrangement fordelaying the local oscillator signal applied to the I/Q demodulator isan amplifier which is optimized to produce a desired delay to a desiredfrequency range, and that the amplifier is preferably a small-signalamplifier. In accordance with the invention, the arrangement fordelaying the local oscillator signal applied to the I/Q demodulator isan amplifier which is optimized to produce a desired delay to a desiredfrequency range, and which amplifier is preferably a small-signalamplifier. By means of such an arrangement, the advantages provided bythe method of the invention can be achieved by a simple structure whichrequires no more than few additional components and which is relativelyinexpensive.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail in connection with thepreferred embodiments with reference to the accompanying drawings, inwhich

FIG. 1 shows a block diagram of a linearization arrangement of a poweramplifier in accordance with the invention,

FIG. 2 shows a circuit diagram of a transistor amplifier stage optimizedto have a delay and in accordance with the invention, and

FIG. 3 shows the input and output matchings, i.e. reflection losses,group delay, i.e. delay, and amplification as a function of frequency ofan amplifier stage of a band-pass type.

DETAILED DESCRIPTION OF THE INVENTION

A linearization arrangement of a power amplifier in accordance with FIG.1 comprises, in outline, an I/Q modulator 10, an I/Q demodulator 11, atleast one power amplifier 13 to be linearized, a local oscillator 14, adelay unit 12 and an antenna 15. The I/Q modulator 10 and the I/Qdemodulator 11 operate on a quadrature modulation principle. It enablestwo independent signals to be combined in a transmitter and to betransmitted on the same transmission band and to be separated again fromeach other in a receiver. The principle of the quadrature modulation isthat two separate signals I and Q (Inphase and Quadrature phase) aremodulated by using the same carrier frequency, but the phases of thecarrier waves deviate from each other in such a manner that the carrierwave of signal Q is 90° behind the carrier wave of signal I. The signalsare summed after the modulation. Signals I and Q can be separated fromeach other on account of the phase difference when a sum signal isdemodulated. To ensure the functionality of the method, the localoscillator signals used by the modulator and demodulator, on the basisof which signals the carrier wave is formed, must be mutually of thesame frequency and in a correct phase.

The baseband I and Q signals containing data are conveyed to the I/Qmodulator 10 in which they are combined and modulated to a finalfrequency (transmission frequency). A final-frequency signal is furtherconveyed to the nonlinear power amplifier 13 in which the signal isamplified. There can be several power amplifier units 13 coupled inseries. After the amplification, the signal is conveyed to the antenna15 to be transmitted.

The amplified, final-frequency signal is sampled after an amplifierstage the power amplifier 13 and conveyed to the I/Q demodulator 11 inwhich the sample signal is demodulated to baseband and I and Q samplesignals are separated from it. The baseband I and Q sample signalsobtained are summed to the actual I and Q signals. The summing of thesample signals to the actual signals predistorts the signals applied tothe I/Q modulator 10 and further to the power amplifier 13 in such amanner that the nonlinearity caused by the power amplifier 13 iscancelled to be as low as possible.

The local oscillator unit 14 produces an oscillator signal which isconveyed to the I/Q modulator 10 and, via the delay unit 12, to the I/Qdemodulator 11. The local oscillator signal applied to the I/Qdemodulator 11 is delayed in the delay unit 12 since the sample signalapplied to the I/Q demodulator 11 and modulated in the I/Q modulator 10,synchronized by the local oscillator 14, is delayed in the poweramplifier 13. To ensure a correct manner of demodulation of the samplesignal, the local oscillator signal applied to the I/Q demodulator andthe sample signal must be in a correct phase with respect to each other.The delay generated to the signal in the power amplifier is thuscompensated by the delay unit 12.

FIG. 2 shows a circuit diagram of a small-signal amplifier optimized tohave a delay and in accordance with the invention, the circuit diagramcomprising, for the sake of clarity, only the most relevant componentsto the invention.

Generating delay in the amplifier is based on the known fact that delayis the derivative of phase as a function of angular frequency. In otherwords, a drastic change of phase in a particular frequency range alsoresults in a great delay in the range. In the simulations and practicaltests performed by the applicant, it has been detected that drasticchanges of phase to a desired frequency range can best be implemented byan amplifier whose input is matched to be of a high-pass type and theoutput is matched to be of a low-pass type.

The operation of the arrangement of FIG. 2 is, in outline, as follows: asignal is inputted via an IN port. The input is arranged to be of ahigh-pass type by means of a matching circuit formed by capacitors C1and C2 and a coil L1. The signal passed through the matching circuitcontrols a transistor T. The transistor T amplifies the signal and theamplified signal proceeds through the matching circuit of the output.The output is arranged to be of a low-pass type by means a matchingcircuit formed by coils L2 and L3 and a capacitor C3. The signal passedthrough the matching circuit of the output is outputted via an OUT port.

The matching circuits of the input and the output are further mutuallyarranged so as to yield an amplifier of a band-pass type. The changescaused by the amplifier in the phase of the signal, and thus also thedelay, are the greater the narrower the band confined by the matchingsof the input and the output. If the input and output matchings arearranged to the same frequency, a peak-like delay results which isextremely great but active in a narrow frequency area. This is notpurposeful, however, and a more preferable result is achieved when theinput matching is provided in the upper edge of a desired frequency bandto be delayed, and the output matching is provided in the lower edge ofthe frequency band. The result is then a steadier delay which is activein a more extensive frequency range. FIG. 3 illustrates a frequencyresponse S21 and delay GD of such an amplifier as a function offrequency. S11 describes the matching, i.e. return loss, of an input ofa high-pass type. The matching of the input is arranged slightly abovethe average frequency about 380 MHz. Correspondingly, S22 describes thematching, i.e. return loss, of an output of a low-pass type. Thematching of the output is arranged slightly below the average frequencyabout 380 MHz. A band in which the group delay, i.e. the delay GD, ofthe amplifier is relatively steady is formed between the matchings ofthe input and the output. The nearer to the average frequency and eachother the matchings of the input and the output are arranged, the higherand sharper the delay curve GD is. The arrangement can also beimplemented to be adjustable by using adjustable components, byreplacing a capacitor C3 by an appropriate capacitance diode, forexample.

If an amplifier of a local oscillator signal is used between the localoscillator 14 and the I/Q demodulator 11, an arrangement in accordancewith FIG. 2 and generating delay is relatively simple and inexpensive toadd to such an existing amplifier. The arrangement can be implemented inmany different ways by using separate components or by integrating thedelay arrangement into an IC circuit, for example, without deviatingfrom the basic idea of the invention.

It is obvious to those skilled in the art that the basic idea of theinvention can be implemented in various ways with progress intechnology. The invention and its embodiments are thus not restricted tothe examples described above but they can vary within the scope of theclaims.

What is claimed is:
 1. A linearization arrangement of a power amplifier,which arrangement comprises an I/Q modulator in which the basebandsignals I and Q containing data are combined and modulated to a finalfrequency, one or more power amplifiers to be linearized and generatingdelay, by which power amplifiers the final frequency signal is amplifiedand which signal is conveyed to an antenna to be transmitted after theamplification, and a sampling arrangement by which the amplified, finalfrequency signal is sampled before the antenna, and an I/Q demodulatorto which said signal sample is applied and in which the signal sample isdemodulated to baseband and I and Q sample signals are separated fromit, and a feedback in which the I and Q sample signals demodulated andseparated from the signal sample are summed to the actual I and Qsignals, and a local oscillator from which a local oscillator signal isconveyed to the I/Q modulator and the I/Q demodulator, whereby anarrangement is provided between the local oscillator and the I/Qdemodulator, by which arrangement the local oscillator signal applied tothe I/Q demodulator is delayed in order to compensate the delaygenerated in the power amplifiers, wherein: the arrangement for delayingthe local oscillator signal applied to the I/Q demodulator is anamplifier which is optimized to produce a desired delay to a desiredfrequency range; the delay generated in the amplifier delaying the localoscillator signal applied to the I/Q demodulator is generated byarranging the amplifier to be band-passing at the frequency of the localoscillator signal; and the matching of the input of the amplifierdelaying the local oscillator signal applied to the I/Q demodulator ishigh-passing and the matching of the output is low-passing.
 2. Alinearization arrangement as claimed in claim 1, wherein thehigh-passing matching of the input of the amplifier is arranged off theaverage frequency of the amplifier in the upper edge of the pass band,and the low-passing matching of the output is arranged in the lower edgeof the pass band in such a manner that the amplifier has a relativelysteady delay in the pass band.
 3. A linearization arrangement as claimedin claim 1, wherein one or more of the components forming the amplifierdelaying the local oscillator signal applied to the I/Q demodulator areadjustable, whereby the delay generated by the amplifier and thebandwidth can be adjusted.
 4. A linearization arrangement as claimed inclaim 1, wherein the amplifier is arranged to produce a delay rangingfrom 5 to 10 ns.
 5. A linearization arrangement as claimed in claim 1,wherein the amplifier producing the delay is a small signal amplifier.